1. Field of the Invention
The present invention relates to a portable refractometer for measuring the refractive index of a liquid.
2. Description of Related Art
Portable refractometers for measuring the index of refraction of a liquid are known in conventional technology. A portable refractometer measures the index of refraction of a liquid from a critical angle at the boundary surface or interface between the liquid and a prism the index of refraction of which is already known. Because the index of refraction of a liquid changes according to the content of soluble material, generally, a portable refractometer is used as a density measuring meter (densitometer) or sugar concentration measuring meter for measuring the density or sugar concentration of a liquid by converting refractive index difference into density or sugar concentration.
FIG. 1 shows the conventional portable refractometer 100 disclosed in Japanese Unexamined Patent Application Publication No. 2003-344283. As light enters a prism 104 from the side of a liquid to be measured 102, the boundary line of brightness-darkness contrast arises in the direction in which light refracted at a critical angle at a boundary surface 106 is output. The index of refraction (or the density or sugar concentration) is obtained as an image of this boundary line which is formed by an objective lens 108 on an optical scale 110 showing the index of refraction (or density or sugar concentration).
As the index of refraction changes in response to environmental temperature, temperature compensation is necessary in order to enable the density or sugar concentration of a liquid to be measured accurately. In the case of the portable refractometer 100 shown in FIG. 1, temperature compensation is performed by, for example, using water as a liquid to be measured 102 and adjusting the position of the vertical direction of the objective lens 108 using an adjustment screw 112 such that the boundary line of brightness-darkness contrast falls on the scale showing 0% of density or sugar concentration.
On the other hand, a variety of portable refractometers that perform the temperature compensation automatically have been developed. FIG. 2 shows a conventional portable refractometer 200 that has an automatic temperature compensation function. The portable refractometer 200 is configured to perform temperature compensation of a measured value by using a planar bimetal 214 and moving an optical scale 210 vertically (in this figure) in response to temperature changes. For this configuration a relatively long planar bimetal 214 extends within a lens barrel 216. A problem arises as when a user holds the refractometer 200 for a long period the planar bimetal 214 undergoes shape transformation in response to the heat of the hand of the user, thereby inhibiting temperature compensation from being performed accurately. Further, a large number of components are required in order to secure the respective ends of the planar bimetal 214 to the optical scale 210 and the lens barrel 216, making assembly difficult as these components and the planar bimetal 214 must be secured in position inside the lens barrel 216. Moreover, as substantial space is required inside the lens barrel 216 this configuration cannot be applied for a small model, portable refractometer.
There are other portable refractometers which moves the objective lens in response to changes in temperature. In comparison to the configuration shown in FIG. 2, this kind of portable refractometer is not affected by the temperature of the hand of the holder. In the same way as the portable refractometer shown in FIG. 2 however, a cantilever arrangement is used in which a long thin planar member that changes in shape in response to changes in temperature supports the objective lens, thus the degree of change of the objective lens in response to changes in temperature is unstable and temperature compensation cannot be performed accurately. Further, in the same manner as the portable refractometer shown in FIG. 2, a large number of components are used for this arrangement making assembly complicated and mitigating against miniaturization.